We have been working on the molecular basis of transformation by Simian virus 40. We have demonstrated that the tumor specific transplantation antigen is related to the tumor antigen. In cells transformed by temperature-sensitive mutants the transformed phenotype is temperature-sensitive. We have examined the distance between origins of DNA synthesis in normal and transformed cells. We have interpreted our results to indicate that the SV40 tumor antigen transforms by acting as a new initiator of DNA synthesis. BIBLIOGRAPHIC REFERENCES: Anderson, J. L., Martin, R. G., Chang, C. and Mora, P.T.: Tumor-specific transplantation antigen is expressed during SV40 lytic infection with wild-type and tsA mutant viruses. Virology 76: 254-262 (1977). Anderson, J. L., Martin, R. G., Chang, C., Mora, P. T. and Livingston, D. M.: Nuclear preparations of SV40-transformed cells contain tumor-specific transplantation antigen activity. Virology 76: 420-425 (1977).